In the previous Halacha we have discussed that one of the works forbidden on Shabbat is reaping. Included in this prohibition is detaching anything that grows from the ground, whether with regards to wheat and barley or anything else which grows from the earth.
The Prohibition to Climb a Tree on Shabbat
Our Sages (Eruvin 100a) decreed that one may not climb a tree on Shabbat, lest one come to detach a piece of the tree which constitutes the forbidden work of reaping on Shabbat. There is no distinction if the tree is dry or moist, for our Sages prohibited climbing any kind of tree on Shabbat. Our Sages likewise prohibited using a tree in any way on Shabbat, such as hanging one’s coat on a tree branch on Shabbat. Thus, if one climbs a tree on Shabbat, one may not come down until the conclusion of Shabbat, unless one has climbed the tree inadvertently (meaning if one was unaware that doing so was forbidden on Shabbat or that today is Shabbat), in which case one may climb off the tree even on Shabbat.
Walking on Grass on Shabbat
Based on the above prohibition of climbing a tree on Shabbat, it seems that walking on grass on Shabbat should likewise be forbidden, for this is similar to climbing a tree. However, this is indeed not the case, for grass is not considered a tree and is merely a plant; there is no prohibition to touch, sit on, or benefit from a plant on Shabbat, for our Sages prohibited this only regarding trees.
Nevertheless, there is another reason to seemingly prohibit walking on grass on Shabbat which is because, as we know, walking on grass many times causes several blades of grass to be plucked from the ground, thus transgressing the prohibition of reaping on Shabbat.
Nevertheless, the Gemara (ibid.) permits walking on grass on Shabbat, for when one does so, one does not intend (or want) to detach anything from the ground. Although it is still forbidden to perform any work on Shabbat even if one does not intend or wish to perform it, this only applies when we are certain that the forbidden work will be performed; however, we cannot say for sure that every step one takes will cause grass to be plucked from the ground. It is therefore permissible to walk on grass on Shabbat, for one does not intend to detach anything. Even if one is certain that after walking for a prolonged amount of time some blades of grass will certainly be detached, it is nevertheless permissible when one does not intend to do so since there is a lack of certainty regarding every single step.
The Mishnah Berura writes that when the grass or weeds are extremely tall, one should take care not to run on them on Shabbat, for doing so will certainly cause them to be plucked from the ground. He adds that it is possible that one must take care not to walk on them briskly either.